No, definitely not restricted to singlespeed. Most road bikes in the 70's and 80's had horizontal dropouts and rear derailers. Some horizontal dropouts have derailer hanger holes, some don't (mainly on cheaper bikes). On the ones without derailer hangers, there's an adaptor thingy that fits over the axle and hangs down to provide a mounting point for the derailer. Sort of like a replaceable derailer hanger on an aluminum mountain bike...



Vertical dropouts are considered better for a geared bike nowadays, because the axle can't move around, so it'll never slip out of place or track imperfectly (assuming the frame is in alignment).